This disclosure relates to estimating ammonia slip using upstream and downstream NOx sensor signals. This disclosure also relates to estimating NOx conversion by the catalyst.
Modern diesel exhaust systems use a selective catalyst reduction (SCR) system to reduce NOx in the exhaust. In one type of SCR system, urea or ethanol reductant is used over a SCR substrate to reduce engine NOx emissions. A NOx sensor placed upstream of the catalyst is used to determine the quantity of reductant needed. A NOx sensor placed downstream of the SCR is used to measure the amount of unconverted NOx exiting the catalyst, monitoring conversion performance. SCR system performance is measured by the amount of reduction of upstream NOx using a model-based control algorithm. That is, known reaction chemistry adsorption and conversion rate embedded in the controller software are used to model the function of the SCR system. The model is used to control reductant injection to give desired reduction, or conversion, of NOx.
There are at least two problems with the above-described control methodology. First, rate-based physical models are sufficiently inaccurate. Second, NOx sensors is significantly cross-sensitive to ammonia. As a result, the downstream sensor cannot distinguish between poor conversion and ammonia emission. This situation can result in an undesirable operating state. Excessive reductant may build-up on the SCR surface, causing generation of ammonia and ammonia emission (“ammonia slip”). Once this operating state occurs, direct assessment of the downstream NOx sensor is inconclusive for determining if poor NOx conversion exists and undesired NOx are being emitted or if ammonia generation is occurring.